It’s the physical insights that it brings which I find useful. So in the coilgun case, for example, the electrical circuit consists of a capacitor C discharged into an inductor L (the coil) to create a pulsed magnetic field. The field pulls the projectile - typically a ferromagnet (think 4" bright steel nail with the head cut off) - into the coil. At that point the field has to be switched off and the nail then continues through the coil, coming out of the far end at a rate of knots.

Hang on, you say. The thing about an inductor is that the current and voltage are 90 degrees out of phase. So averaged over a cycle the electrical circuit does no net work. How, then, can it have imparted kinetic energy to the nail ? The important thing, of course, is that as the nail enters the coil it changes the inductance. So there’s a dL/dt term (calculus) in the equation as well as the basic L. Each time we differentiate a sinusoid we phase-shift it by 90 degrees. It turns out that this brings the current and voltage back into phase and lo-and-behold we can now do electrical work on the projectile. If we look at the things which contribute positively to the dL/dt term we can work to make them bigger so that we transfer more of the energy. But getting the sign right is important (any negative contributions actually slow the projectile down) and for this my calculus would have needed to be up to scratch.

When I got my first job, the UK were on the cusp of switching from imperial to metric. All works to existing properties were measured in imperial and drawn up at a scale of 1/4" per foot. New builds were designed and drawn up at 1:50 scale which was roughly similar. Hence I had to learn to use both systems and have continued to work in both all of my working career.

After leaving the civil engineering side, the next 30 years I spent in mechanical engineering, you would have thought it would be metric only, but of course US made parts/plans and older UK parts were always still in imperial measurements, so I still had to use both.

I can use either system, I just prefer to use imperial when I’m woodworking.